Went to the range today. Checked the essentials before heading out the door - ammo, mags, cleaning kit, and optics have power (two Aimpoint ML2).

I happened to shake both & one rattled. A faint metallic "tick-a-tick-a" sound. Uh oh.

Remove batteries - still hear it. All caps snugged down - still there. Check the LaRue mounts - solid. Pop open lens covers - crap, it's louder!

The culprit was the lock ring in front of the objective lens. I was able to re-tighten it with a derlin pry tool from an iPad screen repair kit. It could be spun freely in either direction. Doesn't appear to affect zero or water resistance. Has anyone else had this happen or lost one in the field?

I know there was a thread in this forum recently with this exact same issue.  I know the guy called aimpoint but I can't recall anything else about what they said or anything.  try looking back within the last month or two and you should find the thread.  

Thanks for the info. I'll see what turns up.

If that red thing under the lock ring is a seal, I would be worried that the nitrogen or argon purging has escaped and that it will fog up or that moisture could enter.